I have a similar issue to you. Although the van is generally smooth when it starts these days, i do have some evenings when it starts very lumpy, and if i hang aroung 1500RPM the engine wobbles and feels exactly like a missfire. I have drawn from a lot of conclusions that the injectors need changing. I am ok with this - except for the price of the new injectors!! I am currently saving up for a new set of four (being skint post Christmas
) Have been quoted at cheapest, £350....
Good luck mate, hope you get it sorted!